Two-Stroke Engineering design improvements

The only moving parts inside a two-stroke engine are the crankshaft, the connecting rod and the piston. This means two-strokes are very simple engines. Because there is a combustion-stroke whenever the piston travels downward, they are capable of producing tremendous power. It is the same simplicity in design, however, that makes a two-stroke engine less fuel-efficient. At the bottom of the power stroke, the transfer ports, which deliver fresh fuel, are open at the same time as the exhaust port. This allows a significant amount of fresh fuel to run straight through the engine without ever being available for power production. Properly designed exhaust systems help minimize the amount of raw fuel loss in the exhaust process, but a two-stroke engine will always waste some fuel.

Two-Stroke Engine

Many producers of two-stroke performance bikes fit them with the exhaust valve systems. A valve is normally situated alongside the exhaust port. To make a two-stroke engine high-powered, the cylinder is given large ports, particularly the exhaust port. The main problem with this is that it makes the engine produce very little power at low RPM (revolution per minute) and have a high fuel consumption. In a race bike, this is not a problem as the engine will be operating at high RPM almost all the time.

However, in a road/commuter/dirt bike, this presents a problem. To give more low RPM power, as well as enable the engine to be able to produce a lot of high RPM power, a power valve system is used. This also smoothens out the power band, apparent in a lot of high powered two-stroke bikes, especially dirt bikes.

Some power valve systems simply restrict the exhaust gases after they have exited the combustion chamber, while others (such as Suzuki AETC) have valves that actually protrude to almost as if they are in the combustion chamber. The AETC system replicates the size and shape of a cylinder with small ports when they are closed giving good low rpm power, and when open replicate an engine with large exhaust port(s) to give good high RPM power. Yet another system opens and closes a sealed cavity in the head; V-TACS uses this system. On the most basic level, the power valve system simply varies the size of the exhaust port, allowing a small port to be used at low RPM's, and a larger port at a higher RPM. On a more complex level, however, many power valve systems change the compression ratio as well, allowing the optimum compression for low and high RPM's.

With the power valve system, there is a "crossover point". This is the RPM whereby the engine produces the same amount of power, whether the power valve is open or closed. By opening the valve, it allows the engine to produce more power at higher RPM; however, if it is open at low RPM, the engine will produce less power (if it is below the crossover point) than when it is closed. To get the most power out of the engine, the power valve must be open just as the crossover point is reached. The same theory is used with adjustable-cam timing on four-stroke engines, such as the Honda's VTEC.

The advantage of these systems over an engine that has a large exhaust port is that at low RPM, the power valve engines will make more power and use less fuel. It will also produce more power at high RPM than an engine with a small exhaust port.

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